The fast growing demand for extremely hard materials results from their numerous applications. Boron carbide, a typical representative of such materials, can be used in armor plates, and as an abrasion resisting material. Most applications of boron carbide require that its density be as high as possible—in other words, the density should be close to the theoretical density (TD). The manufacturing of high density boron carbide is a multistage process, whose final stage is sintering which can be conducted under high pressure or without applying a pressure. Pressureless sintering of materials is more advantageous compared to hot pressing with respect to process cost and ability to organize it in a continuous mode. In the case of a batch mode, pressureless sintering permits a larger scale production.
The common practice to achieve high density of materials (more than 90% TD) is a use of sintering additives. In the case of boron carbide, carbon is used as a sintering aid. The source of carbon may be amorphous carbon (in a form of carbon black, for example) or carbon precursors. In both cases, the boron carbide powder is blended with an additive, then press compacted and sintered. If a carbon precursor is used as an additive, the compacted “green” body should be further heat-treated by pyrolysis or carbonization (prior to sintering) in order to transform the precursor to carbon.
U.S. Pat. No. 4,195,066 discloses a process which requires very fine raw boron carbide powder (grain size <1 μm, specific area 10-50 m2/g) and uses additives in the form of amorphous carbon as well as organic carbon precursor. Today the submicron boron carbide powder is extremely expensive, therefore the application of the said process is limited.
The process disclosed in GB 2,014,193 requires a sub-micron (0.1 μm) boron carbide powder and uses glucose as a carbon precursor.
U.S. Pat. No. 5,720,911 describes a method of making a boron carbide article by sintering which includes mixing boron carbide with an epoxidized resin in solution, drying to form a granulated homogenized mixture, and carbonizing by maintaining the mixture at least two constant temperatures for predetermined times.
U.S. Pat. No. 5,505,899 refers to a process for producing shaped bodies of boron carbide, comprising homogeneously mixing pulverulent boron carbide with at least one pulverulent monocarbide of Ti, Zr, Hf, V, Nb and Ta, sintering at temperatures from 2100° C. to 2250° C., and further densifying by hot isostatic pressing.
JP 07169190 describes a process comprising preparation of a carbon precursor powder from calcined coke, its further drying and mixing with boron-carbide powder, cold isostatic pressing and further carbonization at a temperature less than 2200° C. in a non-oxidizing atmosphere.
The art is always looking for improved processes which may lead to increased density of the final product and which permit to create more complex and stronger shapes of the final product. It is an object of the invention to provide a process which obtains higher density products utilizing a pressureless sintering and thus avoiding expensive hot pressing processes.
It is another object of the invention to provide an improved process for manufacturing high density boron carbide (about 96% TD) by pressureless high temperature sintering.
It is yet another object of the invention to provide an efficient process that permits to create more complex and stronger shapes of the sintered product.
Other objects and advantages of the invention will become apparent as the description proceeds.